1. Technical Field
The present invention relates to a filter apparatus for removing a metal hydroxide precipitate from an alkali metal hydroxide solution. The present invention will be particularly described with reference to the removal of a metal hydroxide precipitate from an alkali metal hydroxide electrolyte circulated through a metal-air battery, although it will be apparent to those skilled in the art that the present invention has other applications, for instance, the removal of a metal hydroxide from a metal etching solution or a metal surface cleaning solution. The present invention is particularly applicable to the removal of an aluminum hydroxide precipitate from a potassium hydroxide electrolyte circulated through an aluminum-air battery.
2. Description of the Prior Art
Published German Application No. 1,325/84-3 discloses a process for surface cleaning aluminum using a dilute alkaline bath. The bath can contain either sodium hydroxide or potassium hydroxide. Dissolved aluminum hydroxide solids accumulate in the bath. To limit the solids content to 10-30 grams per liter, the applicant proposed separating the solids from the bath by filtration and/or decantation. It is indicated in the published application that the precipitation of solids can be facilitated by adding alkaline salts such as sodium carbonate to the bath, and that the state of aggregation of the precipitate is influenced by the presence of complexing agents such as sodium gluconate. In the example in the application, it is indicated that the filtration can be periodic or continuous, but no details concerning the filtration are given. In the example, a sodium hydroxide solution was the alkaline surface cleaning bath.
European Patent Application Publication No. 0114969 discloses a process and reactor for precipitating an aluminate from an alkaline solution by adding an alkali earth metal hydroxide such as calcium hydroxide to the solution. The alkali earth metal hydroxide precipitate is said to be especially useful to reduce the aluminum content of an alkaline electrolyte solution for batteries having aluminum anodes. A preferred electrolyte solution is indicated to be potassium hydroxide. The reactor contains a filter such as glasswool to filter precipitate from the alkaline solution. However, no provision is made for cleaning the glasswool filter.
Prior U.S. Pat. No. 4,136,026 discloses a system for controlling the dissolved aluminum content of a caustic solution for etching aluminum. A portion of the etch solution is introduced into an open-topped reactor and is treated with aluminum hydroxide. The treatment causes the dissolved aluminum to precipitate from the etch solution in a mean residence time of less than about five (5) hours. A portion of the solution is filtered to remove aluminum hydroxide solids using a vacuum drum filter. Precipitate collected on the outside of the drum filter is scraped off of the drum filter and is collected in a separate container. The solution is a sodium hydroxide solution. The system can be characterized as one that is open, i.e., not closed, in that a number of the containers or vessels in the system are open-topped or exposed to atmosphere. This would be unsuitable with many applications including use with an aluminum-air battery.
Prior U.S. Pat. No. 4,719,156, assigned to assignee of the present application, discloses a system for separating a solid particulate discharge from an aluminum-air battery electrolyte effluent on a continuous basis. The effluent is passed to a solids separator which has an impeller operated at a speed sufficient to concentrate solid particulate in an annulus zone in the separator. The concentrated solids are removed from the annulus zone and allowed to settle in a dewatering vessel from which a clean stream is removed. An example in the patent discloses the use of a potassium hydroxide electrolyte. It is indicated in the example that about 86.4% by weight of the heavy particles (greater than ten microns) and 13.6% by weight of the lighter particles (less than ten microns) were concentrated in the separator so as to be separable from the electrolyte. The balance of the mass of particles remained fluidized throughout the electrolyte, but it is indicated in the example that the fluidization of the particles in the electrolyte was sufficiently suppressed for recirculation of the electrolyte back to the aluminum-air battery. One object of the present invention is to achieve a greater percentage separation of small aluminum hydroxide particles than is possible with the system of U.S. Pat. No. 4,719,156. In this respect, the impeller type separator of U.S. Pat. No. 4,719,156 was found to have optimum usefulness in systems where the electrolyte is a sodium hydroxide solution rather than a potassium hydroxide solution, and where the precipitated aluminum hydroxide particles, for reasons not understood, have a larger average particle size.
A disclosure similar to that of U.S. Pat. No. 4,719,156 is contained in U.S. Pat. No. 4,788,111, also assigned to assignee of the present appliction.